1. Field of the Invention
The present invention relates to a digital displacement measuring instrument for measuring a dimension and the like of an object to be measured based on an axial displacement of a spindle. More specifically, the invention relates to a digital micrometer and a digital micrometer head and the like.
2. Description of Related Art
A graduation-reading micrometer (see, for instance, JP-UM-A-54-117168) and a digital display micrometer (see, for instance, JP-A-11-287602) are typically known.
In the graduation reading micrometer, a screw of millimeter specification (e.g., a screw of 0.5 mm pitch: referred to as a millimeter screw hereinafter) is used in a millimeter reading micrometer, and a screw in inch specification (e.g., a screw of 0.635 mm pitch: referred to as an inch screw hereinafter) is used in an inch reading micrometer. A displacement of a spindle can be read by a sleeve graduation axially provided on an outer circumference of a sleeve and a thimble graduation circumferentially provided on an outer circumference of a thimble.
However, such a graduation as used in the graduation reading micrometer is not required in the digital display micrometer since a displacement of a spindle is detected by a rotary sensor and the like and a detected value is converted into a value in millimeter or inch unit before being digitally displayed. Accordingly, in the digital display micrometer, a reading model in millimeter or inch unit can be provided only with a metric screw.
However, there is a potential error in the digital micrometer on account of electric external noise according to used environment. Accordingly, it is often requested to provide a graduation on a digital micrometer for confirmation sake.
In accordance with recent improvement in reliability of sensors, the graduation is not required to provide the same resolution as a graduation-reading micrometer, where it is sufficient in most cases that the digitally displayed value is confirmed to be consistent with the displacement of the thimble (or displacement of the spindle) at several check points.
Under these circumstances, in order to provide a graduation on a digital micrometer, a millimeter screw and an inch screw are typically used respectively for millimeter-reading and inch-reading models, which, however, requires separate production processes to result in increase in production cost.
Similarly, a millimeter graduation and an inch graduation are axially provided on an outer circumference of the sleeve respectively for millimeter-reading and inch-reading models, which also requires separate production processes and accompanies increase in production cost.
Further, in a digital display that can switch a display mode thereof between a millimeter display and an inch display, when only one of millimeter and inch graduations is provided, the displacement of the spindle cannot be checked by the graduation, which hinders operativity.